1. Field of the Invention
The present invention relates to an input pen of a magnetic coupling type data input tablet for inputting characters and graphs.
1. Description of the Prior Art
FIG. 1 is a sectional view showing a conventional input pen with a data input tablet disclosed in for example Laid-Open Japanese Pat. No. 60-134928. In this drawing, 2 designates a detection coil; 4, a lead wire; 5, a switch; 6, a pen case; 7, a shielded wire; 10, a pen shaft; and 11, a magnetic pipe fitted in the tip of the pen shaft 10 projecting out from the lower end of the pen case 6. On the outer surface of this magnetic pipe 11 is wound the above-mentioned detection coil 2. 12, 13 designate electrodes formed on the data input tablet, and 14, input plate of the data input tablet.
Next, the operation of this device will be explained below. In the data input tablet, a plurality of electrodes 12 and 13 intersect with each other at right angles under the surface of the input plate 14, the electrodes 12, 13 intersecting with each other at right angles being disposed electrically separated. The alternating current is supplied to two parallel lines within these electrodes 12 and 13 from outside to generate a magnetic field. When the detection coil 2 is placed in this magnetic field generated by these two lines, there is generated an induced voltage in the detection coil 2. Applying the alternating current to different electrodes in succession can indicate coordinates on the plate surface by the induced voltage in the detection coil. The voltage thus induced in the detection coil 2 is introduced to the switch 5 by the lead wire 4. The switch 5 is turned on as the input pen is pressed against the input plate 14, and the lead wire 4 is connected to the shielded wire 7, which is connected to the external circuit (not illustrated) of the input pen.
The voltage induced in the aforementioned detection coil 2 is determined by magnetic field strength generated at the surface of the input plate 14, the distance from the input plate 14 to the detection coil 2, and the number of turns and winding diameter of the coil.
The input pen of the conventional data input tablet, being of the constitution as stated above, has the problem that a noise is easily detected by the lead wire 4 of the detection coil 2 which extends a substantial length along the pen shaft 10 and furthermore the detection coil 2 which is exposed is likely to be broken because it comes in contact with some other parts during input pen assembling and operation.
Furthermore, there are problems that detecting accuracy is lowered because the magnetic flux generated by other than the electrodes 12 and 13 is detected as induced voltage by the detection coil 2 and an electrostatic noise resulting from capacity coupling between the detection coil 2 and the data input tablet circuit is detected by the detection coil 2.